1. Field of the Invention
The present invention relates in general to dematrixing processors for moving picture experts group-2 (referred to hereinafter as MPEG-2) multichannel audio decoders, and more particularly to a dematrixing processor for an MPEG-2 multichannel audio decoder which is capable of performing a decoding matrix process with respect to a plurality of compositely decoded signals to restore them to their original status.
2. Description of the Prior Art
The MPEG-2 has prescribed the international standards on audio and video signal compression expression methods. Generally, an MPEG-2 audio channel combination includes five channels based on a 3/2 configuration. Namely, the audio channel combination includes three channels of left (referred to hereinafter as L), right (referred to hereinafter as R) and center (referred to hereinafter as C), and two channels of left surround (referred to hereinafter as LS) and right surround (referred to hereinafter as RS). In an MPEG-2 audio decoder, signals of the fivo channels L, R, C, LS and RS are compositely decoded into signals LO, RO, T2, T3 and T4, in which LO signifies the left channel in stereo, RO signifies the right channel in stereo, and T2, T3 and T4 signify three transmission channels for the multichannel signal process. The five compositely decoded signals LO, RO, T2, T3 and T4 must be restored to their original status L.sub.w, R.sub.w, C.sub.w, LS.sub.w and RS.sub.w, in which the subscript "W" signifies the weighting in an audio encoding.
The MPEG-2 layer 2 is the extension of MPEG-2 layer 1. The MPEG-2 layer 1 includes only the left and right channels and the MPEG-2 layer 2 includes three channels in addition to the two channels in the MPEG-2 layer 1. The MPEG-2 layer 2 utilizes the stereo left and right channels LO and RO to allow the user with the MPEG-2 layer 1 system to listen to tho MPEG-2 layer 2 sound. In this case, the stereo left and right channels LO and RO include all the five channel signals in the MPEG-2 layer 2. For this reason, the MPEG-2 layer 2 coder must perform an inter-channel matrixing operation. At this time, channel matrixing information is contained in two parameters, or dematrix procedure (referred to hereinafter as DP) and transmission channel allocation (referred to hereinafter as TC). The MPEG-2 audio decoder performs a dematrixing operation on the basis of the parameters DP and TC as shown in the following Table 1. The dematrixing operation is implemented by the combination of addition and subtraction, The following Table 1 shows a decoding matrix process based on the transmission channel allocation.
TABLE 1 ______________________________________ TRANSMISSION CHANNEL.sub.-- ALLOCATION DECODING MATRIX ______________________________________ 0 L.sub.W = LO-T2-T3 R.sub.W = RO-T2-T4 C.sub.W = T2 LS.sub.W = T3 RS.sub.W = T4 1 C.sub.W = LO-T2-T3 R.sub.W = RO-C.sub.W -T4 L.sub.W = T2 LS.sub.W = T3 RS.sub.W = T4 2 C.sub.W = RO-T2-T4 L.sub.W = LO-C.sub.W -T3 R.sub.W = T2 LS.sub.W = T3 RS.sub.W = T4 3 LS.sub.W = LO-T3-T2 R.sub.W = RO-T2-T4 C.sub.W = T2 LS.sub.W = T3 RS.sub.W = T4 4 L.sub.W = LO-T2-T3 RS.sub.W = RO-T4-T2 C.sub.W = T2 LS.sub.W = T3 R.sub.W = T4 5 LS.sub.W = LO-T3-T2 RS.sub.W = RO-T4-T2 C.sub.W = T2 L.sub.W = T3 R.sub.W = T4 6 C.sub.W = RO-T2-T4 LS.sub.W = LO-T3-C.sub.W R.sub.W = T2 L.sub.W = T3 R.sub.W = T4 7 C.sub.W = LO-T2-T3 RS.sub.W = RO-T4-C.sub.W L.sub.W = T2 LS.sub.W = T3 R.sub.W = T4 0 L.sub.W = LO-T2 + jS.sub.W R.sub.W = RO-T2 - jS.sub.wbp C.sub.W = T2 jLS.sub.W = T3 jRS.sub.W = T4 1 C.sub.W = LO-T2 + jS.sub.W R.sub.W = RO-C.sub.W - jS.sub.wbs L.sub.W = T2 jLS.sub.W = T3 jRS.sub.W = T4 2 C.sub.W = RO-T2 - jS.sub.wbp L.sub.W = LO-C.sub.W + jS.sub.wbp R.sub.W = T2 jLS.sub.W = T3 jRS.sub.W = T4 ______________________________________
In the above Table 1, the signal jS.sub.wbp in the case of dematrix procedure DP="10" is an output signal from a low pass filter with a response characteristic of jS.sub.w =0.5 * (jLS.sub.w +jRS.sub.w). Such a low pass filter is typically a filter finite impulse response (referred to hereinafter as IIR) filter. The IIR filter is adapted to input the average of left and right surround signals and to obtain the present output on the basis of the previous two sample inputs and the previous two sample outputs. The IIR filter has the following transfer function H(z): ##EQU1##
The following Table 2 shows coefficients of the IIR filter based on sampling frequencies.
TABLE 2 ______________________________________ SAMPLING FREQUENCY a.sub.0 b.sub.0 b.sub.1 b.sub.2 ______________________________________ 32 KHz 486 2048 -471 370 44.1 KHz 295 2048 -1394 521 48 KHz 294 2048 -1388 520 ______________________________________
As seen from the above Table 2, the coefficients a.sub.0, b.sub.0, b.sub.1 and b.sub.2 of the IIR filter are different according to the sampling frequencies. Defining x(n) and y(n) respectively as input and output of the transfer function H(z) of the IIR filter in a time domain, the input and output relation can be expressed as follows: ##EQU2##